The present application relates to machine vision vehicle wheel alignment systems configured to measure the alignment of the wheels of a motor vehicle, and in particular, to a machine vision vehicle wheel alignment system configured with a set of imaging sensors disposed in spaced-apart proximity to a heavy-duty motor vehicle having two or more axles in order to observe targets individually mounted to each axle of the vehicle and to the vehicle frame, to determine a set of vehicle wheel alignment angle measurements.
It is well known that improper alignment of the wheels of a vehicle can have adverse effects on vehicle handling, vehicle fuel consumption, and tire wear. The problems associated with misalignment of vehicle wheels are further exacerbated on vehicles having more than two axles, such as heavy-duty trucks, busses, and trailers, which commonly include a steering axle and at least one fixed tandem axle combination.
Various machine vision vehicle wheel alignment systems have been designed to facilitate the measurement and alteration of vehicle wheel alignment angles on light-duty vehicles having two axles, such as light trucks and passenger cars. For example, U.S. Pat. No. 5,675,515 to January utilize machine vision sensors to observe optical targets or identifiable features associated with the vehicle wheels to determine position and orientation in three-dimensional space, from which alignment angles can be mathematically determined. Optical targets utilized by the machine vision vehicle wheel alignment systems can be mounted to the vehicle structure and vehicle wheels in any of a number of known methods, including with the use of wheel adapters configured to grip tire surfaces, wheel rim edges, or both surfaces of a vehicle wheel.
Procedures and methods by which additional vehicle wheel alignment angles such as caster and steering axis inclination can be determined from measurements of the wheel toe angles and camber alignment angles are well known, and are described in SAE Publication 850219 to January, entitled “Steering Geometry and Caster Measurement”. In addition to determining vehicle wheel alignment angles and measurements, it is known to measure the actual distances between vehicle wheels on different axles. When determining alignment angles, such as toe angles, for a vehicle having two axles, the individual toe angles of each vehicle wheel must be defined relative to a longitudinal reference axis. In general, there are two different longitudinal reference axis which are conventional in the vehicle wheel alignment industry.
The reference axis utilized for measurement of the toe angle of the wheels on the rear (fixed) axle of a two-axle vehicle, such as a passenger car or light truck is commonly known as the “geometric centerline” of the vehicle, which is an axis between the center points of the two axles. For the steered vehicle wheels (front axle) of a two-axle vehicle, the individual toe alignment angles are defined to be relative to a reference axis commonly known as the “thrust line”. The “thrust line” is practically determined as the bisector of the total rear toe angle. In essence, the thrust line is determined as the net pointing direction of the rear wheels in a two-axle vehicle, which means that the individual front toe alignment measurements for passenger cars and light trucks are intentionally sensitive to the toe alignment of the rear wheels.
There are a number of practical advantages in determining toe alignment angles relative to these two reference axis for vehicles having two axles. First, the toe angle adjustment of the rear wheels can be accomplished with the front wheels steered only approximately straight ahead. Second, the thrust line thus determined is approximately the line along which the center of the rear axle will travel when the vehicle moves in a straight line, and this line is made to point approximately through the centers of the front and rear axles. Third, the toe adjustment of the front wheels can be accomplished with the steering wheel held straight such that the front toe measurements are symmetric about the thrust line, thereby insuring that the steering wheel is straight when the vehicle moves in a straight line. Fourth, vehicle manufacturers have long provided toe alignment specifications which are relative to these two axes. Any vehicle alignment system which defines toe alignment relative to another axis will not be able to correctly align a vehicle to a manufacturer's specifications.
When aligning the wheels of a heavy-duty vehicle, and in particular, a heavy-duty vehicle having more than two axles, additional reference lines and angles must be measured, determined, and considered to achieve proper wheel alignment for all of the vehicle wheels. The geometric centerline of a heavy-duty vehicle, such as one having more than two axles, is defined as a line drawn through the midpoint of the front axle and the rear reference axle (which is typically the drive axle), as shown in FIG. 1. Correspondingly, a thrust line is defined as the bisector of the total toe angle of the selected reference axle with the thrust angle defined as the angle formed between the geometric centerline and the thrust line. Since vehicles with more than two axles typically have multiple rear axles, there can be multiple thrust lines and thrust angles, depending upon which axle is utilized as a reference axle. This is further complicated in some measurement systems by reference to the vehicle frame, i.e., the longitudinal centerline of the vehicle frame, as further illustrated in FIG. 1.
An angle formed between two thrust lines of a vehicle is referred to as the tandem scrub angle, and is indicative of a misalignment between the axles. The effect of this misalignment is that the wheels on the steer axle of the vehicle must be turned to offset the “push” of the tandem axles to maintain the vehicle moving in a straight-ahead direction, resulting in tire wear on every tire of the vehicle. This condition can be further amplified due to misalignment in the axles of towed trailers, such as in a tractor-trailer combination, resulting in rapid wear on all of the vehicle tires, increased fuel consumption, and poor vehicle handling.
For heavy duty trucks, and vehicles having more than two axles, an alternative reference line based on the centerline of the vehicle frame may be utilized in place of the geometric centerline or driven axle thrust line. For example, European Patent No. 0 757 779 B1 describes a system for measuring wheel angles and chassis unit positions of a vehicle in which a pair of lateral measurement scales are attached to a vehicle at the front in rear in a determined relationship to the vehicle longitudinal axis, and are illuminated/observed by wheel-mounted sensor units to enable the individual axles to be adjusted relative to the vehicle longitudinal axis.
Accordingly, when aligning a heavy-duty vehicle, such as one with more than two axles, a total alignment procedure is required in which every axle on the vehicle is measured and set parallel to an identified reference axis, such that all of the vehicle wheels roll in the same direction, minimizing the vehicle's rolling resistance.
In a conventional alignment procedure for a vehicle having more than two axles, wheel alignment angle sensors containing angle transducers and/or emitters are mounted on the wheels of the steer (front) axle and on one of the tandem drive axles, defined as the reference axle. If necessary, the sensors are compensated for runout in a conventional manner, which typically requires that the vehicle be jacked up or rolled to permit each wheels to be rotated to two or more positions at which measurements are acquired. Once the sensors are mounted and any necessary compensation completed, the rear reference axle is measured to determine a thrust angle. If the rear reference axle is adjustable, it is aligned such that the thrust line and the vehicle frame centerline are collinear. Next, the steer (front) axle of the vehicle is aligned relative to the rear reference axle. Subsequently, the alignment angle sensors are dismounted from the steer (front) axle of the vehicle, and moved to the second rear axle. The sensors are again compensated for runout if necessary, and the second rear axle is aligned relative to the rear reference axle. For other multi-axle vehicle configurations, such as vehicles with two or more steer axles, similar procedures are followed in which the sensors are moved from axle to axle as required to align all axles to a reference axle.
Accordingly, it would be advantageous to provide a vehicle alignment system with the capacity to obtain real-time alignment angle measurements from more than two axles on a multi-axle vehicle, together with any necessary vehicle reference axis measurements, eliminating the repetition associated with the need to reposition alignment angle sensors on multiple axles during the alignment of a multi-axle vehicle.
It would be further advantageous to provide a vehicle alignment system capable of acquiring real-time alignment angle measurements, together with measurements of a vehicle reference axis, using a set of passive optical targets mounted to the vehicle and a set of observing cameras positioned in spaced-apart proximity to the vehicle.